(1) Field of the Invention
The present invention relates to an image reading apparatus with a carriage having mounted thereon a linear light source that emits light to a document sheet to be read. The present invention also relates to an image forming apparatus having the image reading apparatus and a method for positioning the carriage at a home position.
(2) Description of the Related Art
According to one practical configuration of an image reading apparatus for reading the image of a document sheet placed on a platen glass, a linear light source that throws light onto the document sheet is mounted on a carriage. In another configuration, an image sensor that receives light reflected from the document sheet is also mounted on the carriage, along with the linear light source.
Typically, a linear light source is constructed from a linear lamp, such as a xenon lamp or a fluorescent lamp, and mounted on the carriage to be in parallel with the main scanning direction of the image sensor. The carriage includes an image reading motor constructed, for example, from a pulse motor and also includes a power transmission unit constructed from pulleys, belts, and the like. With those components, the carriage is configured to make reciprocating motion below the platen glass in the sub-scanning direction, which is a direction orthogonal to the main scanning direction.
Typically, the carriage is positioned at the home position (the normal standby position) located in the vicinity of the platen glass. To read the image of a document sheet placed on the platen glass, the image reading motor translates the carriage (i.e., moves the carriage uniformly without rotation) from the home position in a first direction parallel to the platen glass. While the carriage is in the translatory motion, the linear light source emits light onto the document sheet and the image sensor receives light reflected from the document sheet. The carriage is first moved in the first direction a predetermined distance corresponding to the size of the document sheet that is placed on the platen glass and then moved back to the home position in a second direction that is the reverse direction of the first direction.
Note that the amount of light emitted by the linear light source mounted on the carriage may not be uniform in the longitudinal direction (sub-scanning direction), which leads to errors such as uneven exposure and uneven image forming on the lens. In order to correct such errors, shading correction is performed. In the shading correction, correction factors necessary for conducting the shading correction are calculated prior to the reading of the image of a document sheet. When the image sensor subsequently acquires the image data of the document sheet, the thus acquired image data is corrected using the calculated correction factor. Generally, an image reading apparatus has a reference pattern member that forms a reference pattern. The reference pattern member is disposed at a location where the reference pattern is readable by the image sensor. The reference pattern is formed with a black reference area and a white reference area that are used for obtaining reference data of the respective colors necessary for conducting the shading correction.
To conduct the shading correction, the carriage is moved from the home position to the position where the reference pattern is readable by the image sensor. At the position, the image sensor receives light reflected from the black and white reference areas of the reference pattern. Then, the carriage is restored to the home position. The correction factors necessary for the shading correction are calculated based on the amounts of reflected light received by the image sensor from the black and white reference areas.
The carriage is normally positioned at the home position. At the time of reading of the image of a document sheet, the carriage is moved a distance necessary to enable the image sensor to read. At the time of the shading correction, the carriage is moved to a position where the reference pattern is detectable by the image sensor. Conventionally, whether or not the carriage is currently located at the home position is detected by a home position sensor as disclosed in JP patent application publication No. 05-188486 (hereinafter, “Patent Document 1”).
Unfortunately, however, the use of a home position sensor as disclosed in Patent Document 1 is not preferable in terms of economy and cost. In addition, the space for accommodating the home position sensor needs to be secured in the vicinity of the moving range of the scanner (carriage), which may lead to another disadvantage that the compactness of the image reading apparatus needs to be compromised. This is more problematic regarding all-in-one MFPs into which an image reading apparatus and an image forming apparatus are integrated. Generally, MFPs are required to realize improved space-saving efficiency in order to reduce the size. It is therefore difficult to secure space for a home position sensor.
JP patent application publication No. 2004-104467 (hereinafter “Patent Document 2”) discloses a configuration according to which no home position sensor is provided but an optically readable detection pattern is provided at the location of the home position. The travel object (carriage) is positioned at the home position according to an electric signal generated as a result of detecting the detection pattern.
Unfortunately, however, the configuration disclosed by Patent Document 2 requires to specifically provide, at the location of the home position, a detection pattern for detecting the home position, which may increase cost. In addition, the space for providing the detection pattern needs to be secured. This is more problematic regarding image reading apparatuses having an automatic document feeder (ADF). Normally, the home position of such an apparatus is located between the position for reading an image of a document sheet transferred by the ADF and the position for reading an image of a document sheet placed on the platen glass. It is therefore difficult to secure the space for providing the detection pattern.
FIG. 11 is a schematic view for illustrating the home position of an image reading apparatus having an automatic document feeder (ADF). The image reading apparatus has a slit formed through the top wall of the housing and at a position corresponding to the position for reading an image of a document sheet transferred by the ADF. A contact glass (slit glass) 81 is fit within the slit and flanked by a contact glass for placing a document sheet thereon. A carriage 84 has a linear light source and an image sensor both mounted thereon. The home position HP of the carriage 84 is set at a location between the slit glass 81 and a platen glass 82. In addition, a reference pattern member 83 having a reference pattern used for shading correction is disposed at a location between the home position HP and the platen glass 82.
As described above, the home position HP is established between the slit glass 81 and the platen glass 82, and the reference pattern member 83 is provided between the home position HP and the platen glass 82. With the above configuration, it is difficult to secure space between the slit glass 81 and the platen glass 82, for additionally providing a sensor and a dedicated detection pattern both for detecting whether or not the carriage is located at the home position.
In order to address the problems noted above, a suggestion has been made regarding the image reading apparatus having the configuration shown in FIG. 11. When the power is turned ON (i.e., at the start of power supply to the image reading apparatus), the image sensor mounted on the carriage 84 detects the boundary between the black reference area and the white reference area of the reference pattern formed on the reference pattern member 83 and designates the detected boundary as a reference position RP. The carriage 84 is then moved with reference to the reference position RP to be positioned at the home position HP.
That is, as schematically denoted by (1) in FIG. 11, in order to enable the image sensor to detect the reference pattern of the reference pattern member 83, the carriage 84 is moved at the power-ON a first predetermined distance in the first direction, which is a sub-scanning direction of the image of a document sheet placed on the platen glass 82. Once the image sensor detects the reference pattern, the carriage 84 is then moved to the reference position RP based on the detected position at which the reference pattern is detected. Subsequently, the carriage 84 is moved a second predetermined distance (the distance from the reference position RP to the home position HP) in the second direction, which is the reverse direction of the first direction. As a result, the carriage 84 is restored to the home position HP.
With the above configuration, if the image sensor fails to detect the reference pattern of the reference pattern member 83 even after the carriage 84 has been moved the first predetermined distance in the first direction, the current location of the carriage 84 cannot be identified. Therefore, the carriage 84 is moved a maximum permissible travel distance in the second direction in order to forcibly bring the carriage 84 to a marginal position 85 of the image reading apparatus in the sub-scanning direction. Subsequently, the carriage 84 is moved in the first direction to enable the image sensor to detect the reference pattern member 83. Once the image sensor detects the reference pattern member 83, the carriage 84 is moved the second predetermined distance into the second direction from the detection position. As a result the carriage 84 is restored to the home position HP.
The above-described configuration, however, involves the following risk. That is, even after the carriage 84 collides against the housing at the marginal position 85 and thus comes to a stop, the motor may keep rotating without being stopped and the driving force into the second direction may be given to the carriage 84. In such a case, a power transmission unit configured to move the carriage 84 is likely to cause trouble. For example, the coupling between the pulley and belt may break away. After such trouble, the power transmission unit is no longer capable of moving the carriage 84. In addition, the collision noise may occur to undesirably make the user feel anxiety or discomfort.
The same trouble may occur when the carriage 84 is located in the vicinity of a side wall 86 of the housing, as denoted by (2) in FIG. 11. The side wall 86 is located away from the slit glass 81 across the platen glass 82. That is, in an attempt to move the carriage 84 in the first direction to enable the image sensor to detect the reference pattern of the reference pattern member 83, the carriage 84 comes to collide against the side wall 86 of the housing before traveling the predetermined distance.